1. Field
The present invention generally concerns a method of forcing a nonlinear circuit (that is, a circuit whose application of a given function varies with amplitude of the input signal) to apply its function to an amplitude-modulated source signal without such nonlinearity. Without modifying characteristics of the circuit itself, this is done by combining the amplitude modulated source signal with one or more dummy signals (one dummy signal for dominantly third order nonlinearities, two for fifth order, etc.) to provide a combined signal that will be treated linearly. The dummy signal(s), and other signals generated upon introduction of the dummy signal, are then filtered from the circuit""s output.
2. Background
A circuit is xe2x80x9clinearxe2x80x9d when it applies the same function to input signals regardless of the input signals"" characteristics. For instance, a circuit is free from amplitude-dependent nonlinearity if it applies the same function to input signals whether they have a small amplitude or a large amplitude. Conversely, a circuit exhibits amplitude-dependent nonlinearity if its function changes according to the amplitude of the input signal. One example of a circuit with amplitude-dependent nonlinearity is an amplifier that multiplies small amplitude input signals by ten, but with input signals of increasing amplitude, multiplies them by successively lesser numbers such 9.8, 9.7, 9.6, 9.5, and so on. The amplifier""s behavior is therefore dependent upon the magnitude of its input signal.
Nonlinearity is an inherent property of many circuits as well as various circuits elements such as transistors, and it may even be desirable in different situations. In processing amplitude-modulated communication signals, however, nonlinear circuit elements are usually undesirable. Amplitude-modulated signals, by definition, express information by the manner in which the amplitude of a signal""s envelope varies. Due to this amplitude-based variation, a nonlinear circuit will process an amplitude-modulated input signal inconsistently; the same function is not applied universally. One effect of this is that the input signal""s frequency bandwidth is broadened. For example, an input signal that initially occupies a narrow frequency bandwidth ends up occupying a wider range of frequencies. Therefore, circuits with amplitude-dependent nonlinearity often increase the bandwidth of amplitude-modulated input signals.
This frequency spreading can cause problems. For example, a communication device""s output signal, broadened by this nonlinear effect described above, may overlap into the frequency channel being used by another device of the same type. As a more particular example, one cordless telephone""s signal may overlap into the frequency channel being used by another cordless telephone. This is called xe2x80x9cinterferencexe2x80x9d and can significantly degrade the other device""s operation. Moreover, if the subject device is using a channel on the edge of the allocated frequency band for such devices, the device""s output signal may even overlap into the frequency band for unrelated devices. Thus, a cordless phone may interfere with a different device that is not even a cordless phone.
Presently, engineers typically try to remove or compensate for nonlinearity in signal transmitters by techniques such as limiting the range of input signals for which a nonlinear circuit is used, and filtering the output of the nonlinear circuit to remove signals of unwanted frequencies. Other techniques are also known, such as predistortion linearization, feedforward linearization, and modulation feedback.
Still, these techniques are not quite adequate in all cases. For instance, problems still exist because predistortion requires an accurate model of the nonlinearity, feedforward requires precise and adaptive matching of RF circuits, and modulation feedback is prone to instability.
A method of linearizing a circuit with amplitude-dependent nonlinearity (xe2x80x9cnonlinear circuitxe2x80x9d) enables the circuit to apply its function without its inherent nonlinearity and without having to modify the circuit""s operating characteristics. This is done by combining the amplitude-modulated source signal with a dummy signal to provide a combined signal that is treated linearly by the nonlinear circuit. The dummy signal and other signals generated by the introduction of the dummy signal are later filtered from the circuit""s output.
According to a more particular aspect of the invention, the following operations are performed. Initially, an amplitude-modulated source signal, which has a source frequency bandwidth and a source envelope, is received. A dummy envelope is computed that would yield a constant if the source and dummy envelopes were to be combined in a predetermined way. An amplitude-modulated dummy signal, which exhibits the computed, dummy envelope and has a prescribed dummy frequency bandwidth different than the source frequency bandwidth, is generated. The source and dummy signals are added to form a combined signal, which is directed to an input of a nonlinear circuit. Signals of the dummy frequency bandwidth and other signals formed by the introduction of the dummy signal are filtered from the output, thereby providing a linearized output attributable solely to the source signal.